1. Field of the Invention
This invention relates to a liquid jet recording head which performs recording by jetting a liquid to form flying liquid droplets.
2. Description of the Prior Art
Ink jet recording methods (liquid jet recording methods) are recently attracting attention for such advantages that generation of noise during recording is negligibly small, high speed recording is possible and recording can be done on so-called plain paper without need of the special treatment of fixing.
Among such methods, the liquid jet recording technique disclosed in, for example, Japanese Laid-open patent application No. 51837/1979, Deutsche Offenlegungsschrift (DOLS) No. 24843064 has a specific feature different from other liquid jet recording methods in that the driving force for discharging liquid droplets is obtained by permitting heat energy to act on a liquid.
More specifically, according to the recording method disclosed in the above patent specifications, liquid which has received action of heat energy undergoes a change in state accompanied with an abrupt increase of volume, and through the acting force based on the change in state is discharged liquid through the orifice at the tip end of the recording head section to be formed into flying liquid droplets, which liquid droplets are attached onto a material to be recorded, thereby effecting recording thereon.
In particular, the liquid jet recording method disclosed in DOLS No. 2843064 is not only applicable very effectively for the so-called drop-on demand recording method, but also can easily be embodied into a recording head in which the recording head portion is made into a high density multi-orifice or full line type, thus being capable of giving images of high resolution and high quality at high speed.
The recording head section of a device that can carry out the above-mentioned method has a liquid discharging portion having an orifice for discharging liquid and a liquid pathway, which is connected to the orifice and has a heat acting portion at which thermal energy acts on liquid for discharging liquid droplets, and an electro-thermal transducer as a means for generating thermal energy.
The electro-thermal transducer has a pair of electrodes and a heat-generating resistance layer which is connected to these electrodes and has a region for heat generation (heat-generating section) between these electrodes.
A typical example exhibiting the structure of such a liquid jet recording head is shown in FIG. 1A, FIG. 1B and FIG. 1C. FIG. 1A is the front view of a liquid jet recording head as seen from the orifice side, FIG. 1B is a partial sectional view of FIG. 1A when cut along the broken line X-Y and FIG. 1C is a plan view of the substrate.
The recording head 100 has a structure having orifices 104 and liquid discharging sections 105 formed by bonding a grooved plate 103 provided with a certain number of grooves of certain width and depth at a predetermined line density to a substrate 102 provided on its surface with an electro-thermal transducer 101 so as to cover the surface of the substrate 102. The recording head shown in the drawing has a plural number of orifices 104. Of course, the present invention is not limited to such an embodiment, but also a recording head with a single orifice is included in the category of the present invention.
The liquid discharging section 105 has an orifice 104 for discharging liquid at its terminal end and a heat acting portion 106 where thermal energy generated from an electro-thermal transducer 101 acts on liquid to generate a bubble and cause an abrupt change in state through expansion and shrinkage of its volume.
The heat acting portion 106 is positioned above the heat-generating section 107 of the electro-thermal transducer 101 and has a heat acting face 108 in contact with the liquid at the heat-generating section 107 as its bottom face.
The heat-generating section 107 is constituted of a lower layer 109, a heat-generating resistance layer 110 provided on the lower layer 109 and a first protective layer 111 provided on the heat-generating resistance layer 110. The heat-generating resistance layer 110 is provided on its surface with electrodes 113 and 114 for current flow through the layer 110. The electrode 113 is common to the heat-generating portions of the respective liquid discharging sections, and the electrode 114 is a selective electrode for selecting the heat generating portion of each liquid discharging section for heat generating and is provided along the liquid pathway of the liquid discharging section.
The first protective layer 111 has the function of separating the heat-generating resistance layer 110 from the liquid filling the liquid pathway of the liquid discharging section for protection of the heat-generating resistance layer 110 chemically or physically against the liquid employed at the heat-generating section 107, and also has the protective function for the heat generating resistance layer to prevent short-circuit through the liquid between the electrodes 113 and 114. The first protective layer 111 also serves to prevent electrical leaks between adjacent electrodes. In particular, prevention of electrical leaks between the respective selective electrodes or prevention of electric corrosion, which will occur by flow of electric current between the electrode under each liquid pathway and the liquid which may happen to come into contact with each other for some cause, is important and for this purpose the first protective layer 111 having such a protective function is provided at least on the electrode existing under the liquid pathway.
Further, the liquid pathway provided at each liquid discharging section is connected upstream thereof to the common liquid chamber (not shown) for storage of the liquid to be supplied to said liquid pathway, and the electrode connected to the electro-thermal transducer provided at each liquid discharging section is generally provided for convenience in designing so that it may pass beneath the aforesaid common liquid chamber on the side upstraem of the heat acting portion. Accordingly, it is generally practiced to provide the upper layer as described above even at this portion in order to prevent contact between the electrode and the liquid.
As described above, on the heat-generating resistance layer 110, there is provided an upper layer 111 for protecting the layer chemically and physically from the liquid employed and also for preventing short-circuits between the electrodes through the liquid. The material for constituting the upper layer 111 should preferably be an organic resin with respect to coating characteristic, but it cannot be used for the heat-generating section due to inferior heat resistance. Accordingly, improvement of heat resistance has been attempted by making the film thicker in forming films of inorganic oxides, metal oxides, etc. which are relatively excellent in thermal conductivity and heat resistance, according to the vapor deposition method, the sputtering method, CVD method, etc.
However, although heat resistance can be improved as the upper layer 111 is made thicker, the thermal energy generated in the heat-generating resistance layer 110 will be lost in the upper layer 111, whereby the heat energy acting on liquid is reduced. Accordingly, for ensuing sufficient energy acting on liquid, the amount of heat generation must be increased, which in turn will result in acceleration of deterioration of the heat-generating resistance layer.
On the other hand, the thickness of the electrodes is determined so as to ensure reliability taking wiring resistance value and other conditions into consideration. The upper layer must have a thickness sufficient to cover the step difference created between the heat-generating section and the section where electrodes are provided. Accordingly, if the thickness of the electrodes is larger, the upper layer must necessarily be thicker, and the upper layer covering over the terminal portions of the electrodes tend to be thinner as shown in FIG. 1B. Such a thin portion may suffer from generation of cracks during energy generation, thus creating a problem in one aspect of durability.